Fracture properties in compression – determination of a compressive fracture energy using digital image correlation technique

The fracture process and damage of concrete in compression is a highly non-localized phenomenon, because numerous micro-cracks occur throughout the entire specimen during loading. In contrast, tensile or flexural failure is characterized by a single macro-crack, for which a conventional “fracture energy” can be provided. For concrete in compression, the situation is more complex, because the type of failure may differ between “crushing”, “splitting” and “spalling”. However, a compressive fracture energy is needed for nonlinear FE calculations e.g. for many numerical degradation models of concrete. Its experimental determination and reliable values are still very scarce. This paper presents the results of several tests performed at the University of Wuppertal on concrete specimens with a height to diameter ratio of 4 to determine the compressive fracture energy of these specimens. The authors used acrylic rods with strain gauges attached and compared the results with measurements using Digital Image Correlation technique (DIC). Compared to the acrylic rods with strain gauges, the DIC system is very robust and more reliable data can be collected. The paper shows new and flexible possibilities offered by DIC to evaluate the compressive fracture energy and to identify the crushed volume without the limitations of conventional measuring methods. So far, DIC systems have not been systematically used to determine compressive fracture energies of concrete. The authors present a method to evaluate compression tests using a DIC system. The compressive fracture energy is determined in a varying number of sections over the entire specimen height. It can be shown that the calculated compressive fracture energy clearly depends on the chosen number of sections. It is demonstrated, that the DIC technology can overcome the inherent problem of conventional measuring techniques, which enforce a predefined partitioning of the specimens. With DIC technology, partitioning can be adjusted during post-processing. However, further tests are needed to show whether this method can be used to determine reliable values for the compressive fracture energy. 1 11th International Conference on Fracture Mechanics of Concrete and Concrete Structures FraMCoS-11 J. M. Chandra Kishen, A. Ramaswamy, S. Ray and R.